Method and Apparatus for Link Sharing Among Multiple EPONS

ABSTRACT

One embodiment of the present invention provides an optical line terminal (OLT) in an Ethernet passive optical network (EPON). The OLT includes a number of bi-directional optical transceivers. At least one bi-directional optical transceiver is coupled to an optical network unit (ONU) group that includes a number of ONUs. The OLT further includes a first downstream media access control (MAC) interface configured to provide a first downstream control signal and a splitter configured to split the first downstream control signal to a number of sub-signals. At least one sub-signal is configured to control downstream transmission of a corresponding bi-directional optical transceiver to a corresponding ONU-group.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/749,285, filed Mar. 29, 2010, which claims the benefit of U.S.Provisional Application No. 61/165,770, entitled “MULTIPLE EPONS SHARINGCOMMON DOWNSTREAM LINK,” filed Apr. 1, 2009. Each of the abovereferenced applications is hereby incorporated by reference in theirentireties.

BACKGROUND

1. Field

This disclosure is generally related to an Ethernet Passive OpticalNetwork (EPON). More specifically, this disclosure is related tomultiple EPONs sharing a common downstream link.

2. Related Arts

In order to keep pace with increasing Internet traffic, networkoperators have widely deployed optical fibers and optical transmissionequipment, substantially increasing the capacity of backbone networks. Acorresponding increase in access network capacity, however, has notmatched this increase in backbone network capacity. Even with broadbandsolutions, such as digital subscriber line (DSL) and cable modem (CM),the limited bandwidth offered by current access networks still presentsa severe bottleneck in delivering large bandwidth to end users.

Among different competing technologies, passive optical networks (PONs)are one of the best candidates for next-generation access networks. Withthe large bandwidth of optical fibers, PONs can accommodate broadbandvoice, data, and video traffic simultaneously. Such integrated serviceis difficult to provide with DSL or CM technology. Furthermore, PONs canbe built with existing protocols, such as Ethernet and ATM, whichfacilitate interoperability between PONs and other network equipment.

Typically, PONs are used in the “first mile” of the network, whichprovides connectivity between the service provider's central offices andthe premises of the customers. The “first mile” is generally a logicalpoint-to-multipoint network, where a central office serves a number ofcustomers. For example, a PON can adopt a tree topology, wherein onetrunk fiber couples the central office to a passive opticalsplitter/combiner. Through a number of branch fibers, the passiveoptical splitter/combiner divides and distributes downstream opticalsignals to customers and combines upstream optical signals fromcustomers. Note that other topologies, such as ring and mesh topologies,are also possible.

Transmissions within a PON are typically performed between an opticalline terminal (OLT) and optical network units (ONUs). The OLT generallyresides in the central office and couples the optical access network toa metro backbone, which can be an external network belonging to, forexample, an Internet service provider (ISP) or a local exchange carrier.The ONU can reside in the residence of the customer and couples to thecustomer's own home network through a customer-premises equipment (CPE).

FIG. 1A illustrates a passive optical network. including an OLT (locatedat a central office) and a number of ONUs (located at customerspremises) coupled through optical fibers and a passive optical splitter(prior art). A passive optical splitter 108 and optical fibers coupleONUs 102, 104, and 106 to an OLT 100. Although FIG. 1 illustrates a treetopology, a PON can also be based on other topologies, such as a logicalring or a logical bus. Note that, although in this disclosure manyexamples are based on EPONs, embodiments of the present invention arenot limited to EPONs and can be applied to a variety of PONs, such asATM PONs (APONs) and wavelength domain multiplexing (WDM) PONs.

FIG. 1B presents a block diagram illustrating the layered structure of aconventional EPON (prior art). The left half of FIG. 1B illustrates thelayer structure of an Open System Interconnection (OSI) model includingan application layer 110, a presentation layer 112, a session layer 114,a transport layer 116, a network layer 118, a data link layer 120, and aphysical layer 122. The right half of FIG. 1B illustrates EPON elementsresiding in data link layer 120 and physical layer 122. EPON elementsinclude a media access control (MAC) layer 128, a MAC″ control, layer126, a logic link control (LLC) layer 124, a reconciliation sublayer(RS) 130, medium interface 132, and physical layer device (PHY) 134.

In an EPON, communications can include downstream traffic and upstreamtraffic. In the following description, “downstream” refers to thedirection from an OLT to one or more ONUs, and “upstream” refers to thedirection from an ONU to the OLT. In the downstream direction, becauseof the broadcast nature of the 1×N passive optical coupler, data packetsare broadcast by the OLT to all ONUs and are selectively extracted bytheir destination ONUs. Moreover, each ONU is assigned one or moreLogical Link Identifiers (LLIDs), and a data packet transmitted by theOLT typically specifies an LLID of the destination ONU. In the upstreamdirection, the ONUs need to share channel capacity and resources,because there is only one link coupling the passive optical coupler tothe OLT.

In order to avoid collision of upstream transmissions from differentONUs, ONU transmissions are arbitrated. This arbitration can be achievedby allocating a transmission window (grant) to each ONU. An ONU deferstransmission until its grant arrives. A multipoint control protocol(MPCP) located in the MAC control layer can be used to assigntransmission time slots to ONUs, and the MPCP in an OLT is responsiblefor arbitrating upstream transmissions of all ONUs coupled to the sameOLT.

Due to the splitting loss at passive optical splitter 108, the number ofONUs coupled to an OLT is limited, thus limiting the number ofsubscribers within a PON. In order to increase the number ofsubscribers, the carrier needs to install more OLTs in the centraloffice. Because OLTs are expensive, it is desirable to find analternative that can allow more subscribers to couple to one OLT.

SUMMARY

One embodiment of the present invention provides an optical lineterminal (OLT) in an Ethernet passive optical network (EPON). The OLTincludes a number of bi-directional optical transceivers. At least onebi-directional optical transceiver is coupled to an optical network unit(ONU) group that includes a number of ONUs. The OLT further includes afirst downstream media access control (MAC) interface configured toprovide a first downstream control signal and a splitter configured tosplit the first downstream control signal to a number of sub-signals. Atleast one sub-signal is configured to control downstream transmission ofa corresponding bi-directional optical transceiver to a correspondingONU-group.

In a variation on this embodiment, the OLT further includes a number ofindividual upstream MAC interfaces, and at least one individual upstreamMAC interface is configured to communicate with a correspondingONU-group.

in a further variation, the individual upstream MAC interface isconfigured to arbitrate upstream transmissions from the ONUs belongingto a corresponding ONU-group.

In a further variation, different individual upstream MAC interfacesseparately arbitrate upstream transmissions from different GNU-groups,thereby facilitating concurrent upstream transmission to the OLT fromONUs belonging to the different ONU-groups.

In a further variation, the individual upstream MAC interfaces areconfigured to allocate discovery slots to respective ONU-groups, whereinthe discovery slots for different ONU-groups are aligned in time.

In a further variation, the downstream transmission and the upstreamtransmissions are carried on two different wavelengths.

In a further variation, the first downstream MAC interface and at leastone individual upstream MAC interface are configured to operate at twodifferent data rates.

In a further variation, the first downstream MAC interface and at leastone individual upstream MAC interface are configured to operate at asame data rate.

In a further variation, the OLT further includes a shared upstream MACinterface configured to interface with more than one of the ONU groups.

In a further variation, the OLT further includes a merger configured tomerge upstream transmissions from the more than one ONU-groups, and tosend the merged transmissions to the shared upstream MAC interface.

In a further variation, upstream transmissions to a respectiveindividual upstream MAC interface and the shared upstream MAC interfaceare carried on two different wavelengths over a single strand of fiber.

In a variation on this embodiment, the OLT further includes an opticaltransmitter and a second downstream MAC interface, wherein the seconddownstream MAC is configured to control downstream transmission of theoptical transmitter to an ONU-group.

In a further variation, downstream transmissions from the bi-directionaloptical transceiver and the optical transmitter are coupled to a singlestrand of fiber via a wavelength division multiplexing (WDM) coupler.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A illustrates a passive optical network including an OLT (locatedat a central office) and a number of ONUs (located at customers'premises) coupled through optical fibers and a passive optical splitter(prior art).

FIG. 1B presents a block diagram illustrating the layered structure of aconventional EPON (prior art).

FIG. 2 presents a block diagram illustrating an OLT that supportsmultiple ONU-groups in accordance with an embodiment of the presentinvention.

FIG. 3 presents a block diagram illustrating an exemplary configurationof an OLT line card for an asymmetric EPON system in accordance with oneembodiment of the present invention.

FIG. 4 presents a block diagram illustrating an exemplary configurationof an OLT line card for an asymmetric EPON system in accordance with oneembodiment of the present invention.

FIG. 5 presents a block diagram illustrating an exemplary configurationof an OLT line card for an asymmetric EPON system in accordance with oneembodiment of the present invention.

FIG. 6 presents a block diagram illustrating an exemplary configurationof an OLT line card for a symmetric EPON system in accordance with oneembodiment of the present invention.

FIG. 7 presents a block diagram illustrating an exemplary configurationof an OLT line card for a symmetric EPON system in accordance with oneembodiment of the present invention.

FIG. 8 presents a block diagram illustrating an exemplary configurationof an OLT line card for a symmetric EPON system in accordance with oneembodiment of the present invention.

FIG. 9 presents a diagram illustrating a WDM-EPON configuration inaccordance with an embodiment of the present invention

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the embodiments, and is provided in the contextof a particular application and its requirements. Various modificationsto the disclosed embodiments will be readily apparent to those skilledin the art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present disclosure. Thus, the present invention is notlimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

FIG. 2 presents a block diagram illustrating an OLT that supportsmultiple groups of ONUs in accordance with one embodiment of the presentinvention. As shown in FIG. 2, an OLT couples to a number of ONU-groupsincluding an ONU-group 210 and an ONU-group 220. ONU-group 210 includesONUs 214-218, all coupled to OLT 200 via a passive optical splitter 212,and ONU-group 220 includes ONUs 224-228, all coupled to OLT 200 via apassive optical splitter 222. Using the architecture shown in FIG. 2,the number of ONUs coupled to an OLT can increase dramatically. Forexample, a conventional OLT may be able to couple to 32 ONUs via apassive optical splitter. With this configuration that includes eightseparate ONU-groups, the number of ONUs coupled to the OLT can reach32×8 (256). To distinguish one ONU from another, each ONU is assignedone or more LLIDs, which are unique across all ONUs coupled to OLT 200.

Similar to a conventional EPON, the downstream traffic is broadcast fromOLT 200 to all ONU-groups including ONU-groups 210 and 220. In otherwords, all ONU-groups share the same downstream link. However, eachONU-group has its own upstream link, and the upstream traffic from ONUsof each ONU-groups is arbitrated separately by its own upstream MACimplementing MPCP located in OLT 200, as explained in more details inthe examples shown in FIGS. 3-9. In other words, OLT 200 is able toarbitrate upstream traffic for each ONU-group separately andconcurrently. As a result, it is possible for two ONUs coupled to thesame OLT to have simultaneous transmission.

MPCP schedules upstream traffic from ONUs via GATE and REPORT messages.MPCP REPORT messages are used by the ONUs to tell the OLT the amount ofdata in its buffer to be sent to the OLT and the MPCP GATE message isused by the OLT to grant a time slot for the ONU to transmit a message.To schedule an ONU's upstream transmission, the OLT sends a GATE messagespecifying receiving LLID and a time slot. As a result, the ONU with thespecified LLID schedules its upstream transmission during the time slotindicated by the GATE message. During a discovery process, in which OLT200 discovers and initializes coupling ONUs, such as ONUs 214-218 and224-228, OLT 200 broadcasts a discovery GATE message to all couplingONUs within different ONU-groups. The discovery GATE message specifies atime interval in which OLT 200 enters the discovery mode and allows ONUsto register (this time interval is called the discovery window). Toregister, ONUs from different ONU-groups can respond to the discoveryGATE message within the discovery window. To avoid collision, themultiple upstream MACs that are responsible for scheduling theirrespective upstream traffic need to synchronize their scheduling of theresponse to the discovery GATE.

Asymmetric EPON

FIG. 3 presents a block diagram illustrating an exemplary configurationof an OLT line card for an asymmetric EPON system in accordance with oneembodiment of the present invention. OLT line card 300 includes a 10Gigabit (G) EPON OLT chip 302, eight optical transceiver modulesincluding module 304 and module 306, an optional packet buffer 308, asynchronous-dynamic random-access memory (SDRAM) 310, and a flash memory312. Packet buffer 308 may include a number of SDRAMs, such as SDRAM332. OLT line card 300 interfaces with a backplane via a redundantuplink interface 314, and a management interface 316. Redundant uplinkinterface 314 can include one or more 10G-attachment-unit-interfaces(XAUIs), and management interface 316 can include an asynchronous busand other Ethernet interfaces. OLT line card 300 couples to eightdownstream ONU-groups, such as ONU-group 326 and ONU-group 328, eachincludes a number of ONUs. Each ONU-group interfaces with OLT line card300 via an optical transceiver module. For example, ONU-group 326interfaces with line card 300 via transceiver module 304.

OLT chip 302 includes an embedded processor 330, a 10G downstream MACinterface 318 that controls the downstream transmission to the eightONU-groups, and eight 1.25G upstream MAC interfaces, such as MACinterface 322 and MAC interface 324, that control the individualupstream transmissions from the eight ONU-groups. The output of MACinterface 318 is split by a 1:8 splitter 320 into eight signals; eachsignal controls the transmission of an optical transceiver module, suchas module 304 and module 306. In other words, all eight opticaltransceivers modules are transmitting the same signal downstream, thusproviding a shared downstream link to all eight ONU-groups. Each of theeight optical transceiver modules has a transmitting port for 10Gdownstream transmission at a wavelength of 1577 nm and a receiving portfor 1.25G upstream receiving at a wavelength of 1310 nm. Because thedownstream transmission and the upstream transmission have differentdata rates, the EPON system is said to be asymmetric. Also note that thedownstream and the upstream signals are carried at differentwavelengths; thus, a single strand of fiber can be used to carry signalsto and from an ONU-group. The upstream receiving of the eight opticaltransceiver modules are independently controlled by eight upstream MACinterfaces, such as MAC interface 322 and MAC interface 324, all workingat a speed of 1.25G. Each upstream MAC interface is configured toarbitrate upstream transmissions from ONUs within an ONU group. As aresult, OLT line card 300 can arbitrate upstream traffic for each ONUgroup separately and concurrently.

In addition to the OLT architecture shown in FIG. 3, other asymmetricalvariations are also possible. For example, in addition to a shareddownstream link, it is also possible for an OLT to provide a dedicateddownstream link to each ONU-group. FIG. 4 presents a block diagramillustrating an exemplary configuration of an OLT line card for anasymmetric EPON system in accordance with one embodiment of the presentinvention. In FIG. 4, OLT line card 400 includes an OLT chip 402, and iscoupled to four individual ONU groups including ONU-group 426 andONU-group 428. In addition to an optical transceiver module, eachONU-group is also coupled to an optical transmitter module. For example,ONU-group 426 is coupled to an optical transceiver module 404 and anoptical transmitter module 408, and ONU-group 428 is coupled to anoptical transceiver module 406 and an optical transmitter module 410. Asa result, in addition to a shared 10G downstream link, OLT line card 400also provides each ONU-group with its own dedicated downstream link.Similar to the one shown in FIG. 3, the shared 10G downstream link iscontrolled by a common 10G downstream MAC interface 412, whose output issent to a 1:4 splitter 416 and is split into four signals, eachcontrolling the downstream transmission of an optical transceivermodule. Optical transmitter modules, such as modules 408 and 410,provide additional dedicated downstream links to individual. ONU-groups.Each optical transmitter module is individually controlled by adedicated downstream MAC interface, thus resulting in a dedicateddownstream link for each ONU-group. For example, optical transmittermodules 408 and 410 are controlled separately by downstream MACinterfaces 422 and 424. Optical transmitter modules 408 and 410 cantransmit at a data rate of 1.25G or 2.5G over a wavelength of 1490 nm.The transmission output of an optical transceiver module (10G, 1577 nm)and the transmission output of a corresponding optical transmitter (1.25or 2G, 1490 nm) can be coupled to a single strand of fiber using awavelength-division-multiplexing (WDM) coupler, such as WDM coupler 430and 432. Providing dedicated downstream links in addition to a shareddown stream link makes it possible for implementing quality of service(QOS) control. Similar to FIG. 3, the 1.25G, 1310 nm upstream link foreach ONU-group is provided by the receiving port of a correspondingoptical transceiver module, which is controlled by an individual 1.25Gupstream MAC interface, such as MAC interface 418 and MAC interface 420.

FIG. 5 presents another exemplary configuration of a 10G OLT line cardfor an asymmetric EPON system in accordance with an embodiment. In FIG.5, OLT line card 500 is coupled to two ONU-groups 526 and 528, providingeach a shared 10G downstream link at 1577 nm, a dedicated 2.5Gdownstream link at 1550 nm, and a 1.25G dedicated downstream link at1490 nm. Similar to FIGS. 3 and 4, the output of a common 10G downstreamMAC interface 512, which is located on an OLT chip 502, is split in twoways by a 1:2 splitter 516, and the split signals control downstreamtransmissions of optical transceiver modules 504 and 506, thus providinga shared 10G downstream link to ONU-groups 526 and 528. In addition,optical transmitter modules 508 and 510, which are separately controlledby 1.25G downstream MAC interfaces 522 and 524, provide dedicateddownstream links at a wavelength of 1490 nm to ONU-groups 526 and 528,respectively. Moreover, optical transmitter modules 538 and 540, whichare separately controlled by 2.5G downstream MAC interfaces 534 and 536,provide additional dedicated downstream links at a wavelength of 1550 nmto ONU-groups 526 and 528, respectively. The three downstreamtransmissions at different wavelengths are multiplexed together by a WDMmultiplexer, such as multiplexers 530 and 532, to a single strand offiber before reaching the passive optical splitters for a correspondingONU-group. Similar to FIGS. 3 and 4, the 1.25G, 1310 nm upstream linksfor each ONU-group are provided by the receiving ports of opticaltransceiver modules 504 and 506.

Symmetric EPON

In addition to asymmetric EPON solutions, embodiments of the presentinvention also include symmetric EPON solutions, where the downstreamand upstream transmissions have the same bandwidth. FIG. 6 presents ablock diagram illustrating an exemplary configuration of an OLT linecard for a symmetric EPON system in accordance with one embodiment ofthe present invention. Similar to FIG. 3, OLT line card 600 couples toeight ONU-groups, such as ONU-group 626 and ONU-group 628, via eightoptical transceivers modules, such as transceiver modules 604 and 606. Acommon 10G downstream MAC interface 618 controls the transmission of alleight transceiver modules via a 1:8 splitter 620, thus providing ashared 10G downstream link at a wavelength of 1577 nm to all ONU-groups.In FIG. 6, the upstream transmission links from each ONU-group include a10G transmission link at a wavelength of 1270 nm and a 1.25Gtransmission link at a wavelength of 1310 nm. Both transmissions arereceived by the receiving port of the corresponding transceiver module,which is capable of dual-rate receiving. For example, the receiving portof transceiver module 604 receives the two upstream transmissions fromONU-group 626. To avoid collisions of the two upstream transmissions,each transmission is assigned a time-division-multiple-access (TDMA)time slot. Similar to OLT chip 302 shown in FIG. 3, OLT chip 602includes a group of 1.25G upstream MAC interfaces, such as upstream MACinterfaces 622 and 624, each controls and processes a 1.25G, 1310 nmsignal received from each individual ONU-group, thus providing adedicated upstream link to each ONU-group. Such dedicated upstream linkallows 1.25G, 1310 nm upstream traffic from each ONU-group to bescheduled separately and concurrently. As a result, it is possible tofor two ONUs coupled to OLT line card 600 to have simultaneous upstreamtransmission. Note that the summed bandwidth of all eight dedicatedupstream links can be 10G. In one embodiment, each dedicated upstreamMAC interface, such as MAC 622 and MAC 624, has a flexible capacity ofup to 10G. Thus, it is possible for an individual ONU-group to have a10G upstream transmission on such a dedicated link. However, theaggregate uplink bandwidth is limited by an aggregate shaper to a sum of10G among all ONU-groups. Consequently, a switch behind the MACs onlysees a limited bandwidth of 10G.

On the other hand, the 10G, 1270 nm upstream transmission from allONU-groups are merged together by a 8:1 merger 610, and the mergedsignal is sent to a common 10G upstream MAC interface 608 for controland processing. As a result, in addition to dedicated upstream links,OLT line card 600 also provides a shared upstream link to allONU-groups. The 10G upstream transmissions from all ONUs within allONU-groups are arbitrated by the MPCP implemented in the common 10Gupstream MAC interface 608.

FIG. 7 presents a block diagram illustrating an exemplary configurationof an OLT line card for a symmetric EPON system in accordance with oneembodiment of the present invention. In FIG. 7, the structure of OLTline card 700 is similar to that of OLT line card 400 illustrated inFIG. 4. In FIG. 7, OLT line card 700 includes an OLT chip 702 andcouples to four ONU-groups including ONU-groups 726 and 728. OLT linecard 700 provides a 10G, 1577 nm shared downstream link and a 1.25G (or2.5G), 1490 nm dedicated downstream link to each ONU-group. The 10Gshared downstream link is provided by a common 10G downstream MACinterface 712, which controls the downstream transmission of opticaltransceiver modules, such as modules 704 and 706, via a 1:4 splitter716. Dedicated downstream links to individual ONU-groups are provided bydedicated 1.25G (or 2.5G) downstream MAC interfaces, such as MACinterfaces 722 and 724, each controlling the transmission of an opticaltransmitter module, such as transmitter modules 708 and 710. The sharedand dedicated downstream transmissions to each ONU-group are coupled toa single strand of fiber via a WDM coupler, such as coupler 730 and 732.The difference between the system shown in FIG. 7 and the one shown inFIG. 4 is that, in addition to a dedicated 1.25G, 1310 nm upstream link,each ONU-group in FIG. 7 is also provided with a 10G, 1270 nm sharedupstream link. Both the dedicated upstream transmission and the sharedupstream transmission from each ONU-group are received by the receivingport of an optical transceiver module, such as modules 704 and 706, andeach upstream transmission occupies a TDMA time slot. The 10G, 1270 nmupstream transmissions received from all four ONU-groups are mergedtogether by a 1:4 merger 736, and the merged signal is sent to a common10G upstream MAC interface 734 for control and processing. As a result,common 10G upstream MAC interface 734 arbitrates the 10G upstreamtransmissions from all ONUs within all four ONU-groups.

FIG. 8 presents a block diagram illustrating an exemplary configurationof an OLT line card for a symmetric EPON system in accordance with oneembodiment of the present invention. In FIG. 8, the structure of OLTline card 800 is similar to that of OLT line card 500 illustrated inFIG. 5. In FIG. 8, OLT line card 800 includes an ONU chip 802 andcouples to two ONU-groups 826 and 828. OLT line card 800 provides a 10G,1577 nm shared downstream link, a 1.25G, 1490 nm dedicated downstreamlink, and a 2.5G, 1550 nm dedicated downstream link to each ONU-group.The 10G shared downstream link is provided by a common 10G downstreamMAC interface 812, which controls the downstream transmission of opticaltransceiver modules 804 and 806, via a 1:2 splitter 816. 1.25 G, 1490 nmdedicated downstream links to individual ONU-groups are provided bydedicated 1.25G downstream MAC interfaces 822 and 824, which control theoptical transmitter modules 808 and 810. 2.5 G, 1550 nm dedicateddownstream links to individual GNU-groups are provided by dedicated 2.5Gdownstream MAC interfaces 838 and 840, which control the opticaltransmitter modules 842 and 844. The shared and dedicated downstreamtransmissions to each GNU-group are multiplexed to a single strand offiber via a WDM multiplexer, such as multiplexers 830 and 832. Thedifference between the system shown in FIG. 8 and the one shown in FIG.5 is that, in addition to a dedicated 1.25G, 1.310 nm upstream link,each GNU-group in FIG. 8 is also provided with a 10G, 1270 nm sharedupstream link. Both the dedicated upstream transmission and the sharedupstream transmission from each GNU-group are received by the receivingport of an optical transceiver module, such as modules 804 and 806, andeach upstream transmission occupies a TDMA time slot. The 10G, 1270 nmupstream transmissions received from both GNU-groups are merged togetherby a 1:2 merger 836, and the merged signal is sent to a common 10Gupstream MAC interface 834 for control and processing. As a result,common 10G upstream MAC interface 834 arbitrates the 10G upstreamtransmissions from all ONUs within the two ONU-groups 826 and 828.

WDM EPON

The systems illustrated in FIGS. 6-8 rely on TDMA for schedulingtransmissions between the shared upstream link and the dedicatedupstream link. It is also possible to utilize a WDM demultiplexer thatdemultipiexes the two upstream links and sends them to two separatereceivers. FIG. 9 presents a diagram illustrating a WDM-EPONconfiguration in accordance with an embodiment of the present invention.In FIG. 9, two OLT line cards 900 and 902 are coupled to eightGNU-groups, such as GNU-group 918. OLT line cards 900 includes a 10GEPON OLT chip 904, which is coupled to the eight ONU-groups via eightoptical transceivers, such as transceivers 908 and 910. OLT line card900 provides a 10G, 1577 nm shared downstream link and a 10G, 1270 nmshared upstream link to each of the eight GNU-groups. OLT line card 902includes a 1G EPON our chip 906, which is also coupled to the eightGNU-groups via eight optical transceivers, such as transceivers 912 and914. Instead of shared links, OLT line card 902 provides each of theeight GNU-groups with a dedicated 1.25G (or 2.5G), 1490 nm downstreamlink and a. dedicated 1.25G, 1310 nm upstream link. The shared and thededicated links are combined using a WDM multiplexer/demultiplexer. Forexample, a WDM multiplexer/demultiplexer 916 combines the 10G shareddownstream transmission from transceiver 910 together with the dedicated1.25G (or 2.5G) downstream transmission from transceiver 912, and sendsthe combined signal to ONU-group 918. Similarly,multiplexer/demultiplexer 916 demultiplexes the upstream transmissionsfrom ONU-group 918 by sending the 10G shared upstream transmission totransceiver 910 and the 1.25G dedicated upstream transmission totransceiver 912.

The data structures and code described in this detailed description aretypically stored on a computer-readable storage medium, which may be anydevice or medium that can store code and/or data for use by a computersystem. The computer-readable storage medium includes, but is notlimited to, volatile memory, non-volatile memory, magnetic and opticalstorage devices such as disk drives, magnetic tape, CDs (compact discs),DVDs (digital versatile discs or digital video discs), or other mediacapable of storing computer-readable media now known or later developed.

The methods and processes described in the detailed description sectioncan be embodied as code and/or data, which can be stored in acomputer-readable storage medium as described above. When a computersystem reads and executes the code and/or data stored on thecomputer-readable storage medium, the computer system performs themethods and processes embodied as data structures and code and storedwithin the computer-readable storage medium.

Furthermore, the methods and processes described above can be includedin hardware modules. For example, the hardware modules can include, butare not limited to, application-specific integrated circuit (ASIC)chips, field-programmable gate arrays (FPGAs), and otherprogrammable-logic devices now known or later developed. When thehardware modules are activated, the hardware modules perform the methodsand processes included within the hardware modules.

The foregoing descriptions of various embodiments have been presentedonly for purposes of illustration and description. They are not intendedto be exhaustive or to limit the present invention to the formsdisclosed. Accordingly, many modifications and variations will beapparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention.

What is claimed is:
 1. An optical line terminal (OLT) in an Ethernetpassive optical network (EPON), comprising: a plurality of opticaltransceivers, each optical transceiver from among the plurality ofoptical transceivers being configured to transmit a downstream signal toa corresponding optical network unit (ONU) group from among a pluralityof ONU groups, each ONU group from among the plurality of ONU groupsincluding a plurality of ONUs; a downstream media access control (MAC)interface configured to provide a downstream control signal; a splitterconfigured to split the downstream control signal into a plurality ofsub-signals, a sub-signal from among the plurality of sub-signalscontrolling transmission of the downstream signal; and a plurality ofupstream MAC interfaces configured to arbitrate a plurality of upstreamsignals transmitted from the plurality of ONUs to the OLT.
 2. The OLT ofclaim 2, wherein the plurality of ONU groups include a first ONU groupand a second ONU group, and wherein the plurality of upstream signalsare transmitted concurrently from the first and second ONU groups to theOLT.
 3. The OLT of claim 2, wherein the downstream signal propagates ona first wavelength, and wherein the plurality of upstream signalspropagate on a second wavelength.
 4. The OLT of claim 1, wherein thedownstream MAC interface and an upstream MAC interface from among theplurality of upstream MAC interfaces are configured to operate atdifferent data rates.
 5. The OLT of claim 1, wherein the downstream MACinterface and an upstream MAC interface from among the plurality ofupstream MAC interfaces are configured to operate at an identical datarate.
 6. The OLT of claim 1, wherein each optical transceiver from amongthe plurality of optical transceivers is further configured to transmitan identical downstream signal to provide a downstream link shared amongthe plurality of ONU groups.
 7. The OLT of claim 1, wherein thedownstream signal and an upstream signal from among the plurality ofupstream signals are transmitted via an optical fiber strand.
 8. The OLTof claim 1, wherein each upstream MAC interface from among the pluralityof upstream MAC interfaces is further configured to independentlycontrol a corresponding optical transceiver from among the plurality ofoptical transceivers to arbitrate the plurality of upstream signals. 9.An Ethernet passive optical network (EPON) system, comprising: a firstoptical line terminal (OLT) configured to transmit a shared downstreamsignal to a plurality of optical network unit (ONU) groups, and toreceive a shared upstream signal from the plurality of ONU groups; asecond optical line terminal (OLT) configured to transmit a dedicateddownstream signal to an ONU group from among the plurality of ONUgroups, and to receive a dedicated upstream signal from the ONU group;and a wavelength division multiplexer/demultiplexer configured totransmit a multiplexed downstream signal to the ONU group bymultiplexing the shared downstream signal and the dedicated downstreamsignal, and to demultiplex a multiplexed upstream transmission receivedfrom the ONU group to provide the shared upstream signal and thededicated upstream signal.
 10. The EPON system of claim 9, wherein theshared downstream signal propagates on a first wavelength, and whereinthe dedicated downstream signal propagates on a second wavelength. 11.The EPON system of claim 9, wherein the shared upstream signalpropagates on a first wavelength, and wherein the dedicated upstreamsignal propagates on a second wavelength.
 12. The EPON system of claim9, wherein the shared downstream signal propagates on a firstwavelength, wherein the dedicated downstream signal propagates on asecond wavelength, wherein the shared upstream signal propagates on athird wavelength, and wherein the dedicated upstream signal propagateson a fourth wavelength.
 13. The EPON system of claim 9, wherein thefirst OLT is further configured to transmit the shared downstream signaland to receive the shared upstream signal via a shared opticaltransceiver.
 14. The EPON system of claim 9, wherein the second OLT isfurther configured to transmit the dedicated downstream signal and toreceive the dedicated upstream signal via a dedicated opticaltransceiver.
 15. The EPON system of claim 9, wherein the first OLT andthe second OLT are further configured to operate at different datarates.
 16. In an optical line terminal (OLT), a method comprising:transmitting a plurality of downstream signals to corresponding opticalnetwork unit (ONU) groups including a plurality of ONUs; generating adownstream control signal; splitting the downstream control signal intoa plurality of sub-signals; and controlling the transmitting of theplurality of downstream signals utilizing the plurality of sub-signals.17. The method of claim 16, wherein the transmitting comprises:transmitting the plurality of downstream signals on a first wavelength,and further comprising: arbitrating upstream transmissions received on asecond wavelength from the plurality of ONUs.
 18. The method of claim16, wherein the transmitting comprises: transmitting the plurality ofdownstream signals at a first data rate, and farther comprising:arbitrating upstream transmissions received at a second data rate fromthe plurality of ONUs.
 19. The method of claim 16, wherein thetransmitting comprises: transmitting the plurality of downstream signalsat a first data rate, and further comprising: arbitrating upstreamtransmissions received at the first data rate from the plurality ofONUs.
 20. The method of claim 16, wherein each downstream signal fromamong the plurality of downstream signals is an identical downstreamsignal, and further comprising: providing a downstream link shared amongthe plurality of ONU groups based on the identical downstream signal.